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Saturday, 20 September 2014

Extra High Voltage measurement using Capacitor Voltage Transformer

In power sub-stations, and testing laboratories, it is necessary to measure the extra high voltages with accuracy. While measuring these enormous voltages, it is essential to ensure the safety of working personnel and related equipments. Capacitor
Voltage Transformer (CVT) or Capacitance Coupled Voltage Transformer (CCVT) is
typically single-phase devices used for measuring voltages exceeding 100 kV and
where the use of Potential Transformers (PT) becomes uneconomical. CVT or CCVT
convert the transmission class voltages for the purpose of metering and
protection or in other words these devices isolate the measuring instruments,
meters, relays etc from High Voltage (HV) or Extra High Voltage (EHV) circuit
or transmission line and provide a low value replica of the original voltage.
CVT also serves as coupling capacitors for coupling high frequency Power Line Carrier
(PLC) signals to the transmission lines.

Construction of Capacitor Voltage Transformer:

CVT
consists of series connected capacitor elements housed in a hermitically sealed
porcelain or silicon rubber shells. These capacitors acts as potential dividers
and steps down the voltage to be measured to an intermediate value of about 5
to 12 kV. This voltage is fed to an electromagnetic circuit consisting of an
auxiliary transformer which gives the final reduced secondary voltage (say 110
V). The dielectric of the capacitor is made up of polypropylene film or paper
impregnated with synthetic fluid. Stainless steel bellows are provided in each
capacitor section and these bellows allow for the expansion and contraction of
the insulating fluid taking place because of the change in the operating
temperature.

CVT
has at least 4 terminals; one connected to the HV or EHV terminals, second to
the ground terminal and two secondary terminals which are connected to the
measuring instruments or protective relays as shown in the figure below. Many renowned manufacturers
including Siemens make an entire range of Instrument Transformers from 72.5 kV
to 800 kV class consisting of Current Transformers (CT), Potential Transformers
(PT), Capacitor Voltage Transformer (CVT) and Combined CT and PT units.

The
main advantages of CVT are:

·High
accuracy because of highly stable capacitance.

·Maintenance-free
performance throughout the life.

·Robust
and reduced size.

·Reliable
ferro-resonance suppression system.

The 1200 kV test station at Bina (M.P.) has installed 1200 kV CVT manufactured by Siemens at its Aurangabad plant.

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About Me

Hello, I am Manoj Nair, specialized in Heavy Electrical Equipment (M-Tech). Also Life Member of ISTE and Associate Member of IE(India).

Currently I am working as Consultant in the field of Electrical Distribution and Energy Conservation. Also worked as Faculty in Electrical Engg, engaged in teaching post graduate and under graduate students of Electrical & Electronics engineering. I have authored/co-authored 8 books on various subjects of electrical engineering.

Also worked in Electric Utility as an engineer and handling the various jobs related to installation and maintenance of distribution lines, transformers and the metering equipments.

In the free time I switch over to blogging, discussions related to power system, developing MATLAB coding for problems related to power system.